Micro-Electrical-Mechanical-Systems (MEMS) such as sensors can be widely used in applications such as automotive, household appliance, building ventilation, and in general industrial applications to sense a physical condition such as pressure, temperature, or acceleration, and to provide an electrical signal representative of the sensed physical condition. Conventional pressure sensor is constructed as a network of resistors in a resistive bridge configuration, wherein the resistive bridge has two terminals for coupling to power supply potentials and two terminals for providing a differential output signal.
A drawback of resistive bridge type sensor is that they produce a non-zero output electrical signal (i.e., offset voltage) at their output terminals due to package stresses. Temperature Coefficient of Offset (TCO) is a measure of non-pressure induced stresses as a function of temperature that is placed on a semiconductor device such as MEMS device and is expressed in microvolts per degree Celsius.
In one prior art a non-zero TCO in a semiconductor is adjusted by reducing the amount of adhesive material utilized to secure a first structure to a second structure. An adhesive layer utilized to secure a sensor die to a constraint die in a pressure sensor application is reduced in thickness and/or formed so that adhesive material does not completely cover the constraint die. The TCO is further adjusted by reducing the amount and/or patterning the adhesive layer employed to secure the sensor to its package.
In another prior art, a structure and method of making a piezoresistive transducer with reduced offset current are disclosed. The transducer is comprised of a piezoresistive die having a support rim and a diaphragm, and a support housing having a wall and an aperture. The shape of the diaphragm is matched with the shape of the aperture while the shape of the supporting rim is matched with the shape of the wall. By matching these shapes, temperature induced stresses are reduced, thus reducing temperature induced offset currents.
Another prior art includes a stress sensitive microchip on a package with an isolator between the microchip and the package. The isolator has a modulus of elasticity that has a relationship with the package's modulus of elasticity. This relationship causes no more than a negligible thermal stress to be transmitted to the microchip.
Referring to FIG. 1, when a pressure die 102 is attached to a package 101, the Coefficient of Thermal Expansion (CTE) mismatch between the different materials produce package stresses that lead to an offset signal on the pressure die 102. Most of these stresses are produced at the interface between the die 102 and the package 101. Temperature Coefficient of Offset (TCO) can be a non-pressure induced signal as a function of temperature on a pressure sensor. To minimize the TCO a backing plate 103 with a CTE close to Silicon can be placed between the Silicon die 102 and the package. 101. The backing plate 103 isolates the sensing die 102 from the package stresses. The Room Temperature Vulcanizing (RTV) 104 or other die attach material for example having a CTE of 4.4e-5 1/C can be used to attach the backing plate 103 to the package 101. Backing plate 103 can be made of Pyrex, Hoya, Borofloat, Silicon or other material with a CTE close to Silicon's CTE. The packages 101 for example can have a CTE of 1.6e-5 1/C.
Usually the backing plate's thickness 102 is increased to minimize the package stresses on the Silicon die 102. Package size, assembly processes and cost limitations limit the thickening of the backing plate. It is well known in the industry that increasing the backing plate's thickness improves its stress isolation properties. However due to space limitation and electrical connection processes it is often not possible for the thickness to be increased.
Based on the foregoing, it is believed that a need exists to modify the backing plate's geometry for improving the stress isolation without increasing its thickness.